1. Field of the Invention
This invention relates to a brushless motor and more particularly it relates to a brushless motor adapted to reduce the cogging torque thereof.
2. Related Art Statement
Brushless motors are used abundantly in various industrial fields in recent years because of the advantage that they do not have any mechanical contact point like between the brush and the commutator and therefore they are free from various problems, such as wear of a brush and electric noises. Examples of fields of application of brushless motors include industrial machines such as robots, IT devices such as personal computers and electric power steering (to be referred to as EPS hereinafter) devices for automobiles. Brushless motors are being increasingly used in these fields particularly due to the development of semiconductor control circuits. However, a so-called cogging torque can easily arise in brushless motors due to the attractive force between the permanent magnet at the rotor side and the core teeth at the stator side. The cogging torque can cause noises and vibrations and, in the case of the EPS device of an automobile, it can degrade steering feeling to the driver of the automobile.
A conceivable measure for reducing the cogging torque is the use of a stator having a multiple of slots in order to level off the unevenness of the torque of the brushless motor. However, it is not possible to infinitely increase the number of slots and hence the effect of a multi-slot stator is limited by the size of the motor. Techniques of using a pseudo-multi-slot stator obtained by forming a groove at the front end of each core tooth where the magnetic flux shows a high density so as to make the core tooth appear like a fork have been proposed as a breakthrough in the limit. For instance, Japanese Utility Model Application Publication No. 7-47981 describes a brushless motor having core teeth that are provided with respective auxiliary grooves arranged at a pitch equal to ½ of the slot pitch so as to produce an effect of doubling the member of slots.
However, as shown in FIGS. 6 and 7, either the magnet 51 and the stator core 52 are made to have a same length or the magnet 51 is made longer than the stator core 52 for the purpose of raising the output. Therefore, if a pseudo-multi-slot stator is realized by forming auxiliary grooves, the magnetic flux partly turns around the end faces 52a of the stator core 52 to lower the effect of the arrangement of auxiliary grooves. In other words, part of the magnetic flux does not pass through the front ends of the teeth and the cogging reducing effect of the pseudo-slots is not fully achieved because of the part of the magnetic flux unrelated to the auxiliary grooves. Measures for improving the effect of reducing the cogging torque by the arrangement of auxiliary grooves need to be taken.
It is therefore the object of the present invention to provide a brushless motor that can improve the effect of reducing the cogging torque by the arrangement of auxiliary grooves.